In drilling for oil and other subterranean resources, generally a rotary drilling system is used in which a drill string is composed of pipe members which are successively linked to the string and lowered into a bore hole as drilling progresses. Generally, the drill string has to be removed out of the bore hole several times which involves disconnecting and reconnecting the pipe members forming the drill string. The need of connecting pipes also occurs when a casing is brought into bore hole.
Usually the pipe members each have a threaded, conical pin and a threaded box of a corresponding shape and size. Subsequent pipe members are connected to each other by screwing the pin of a subsequent pipe member into the box of an upper pipe member of a pipe string. Typically, connecting two pipe members requires careful positioning of the pin into the box to avoid damage to the threads, 5 to 10 complete rotations of the pin relative to the box and accurate control of the applied make-up torque. During the time required to carry out these operations, the drill string is not rotating. Particularly during drilling the interruption caused by the operation of connecting pipes is disadvantageous, because it allows mud in the drilling hole to settle. Due to this effect, a return to a steady drilling equilibrium is more difficult and takes more time the more the conditions in the drilling hole are allowed to diverge from the steady state drilling conditions.
Another disadvantage of such connections is that the conical design of the box and the pin reduces the size of the cross section in the coupling available to allow the passage of mud.
In spite of substantial efforts to provide couplings which are easier to operate and which allow to make connections more quickly, the above type of connections have remained the most widely used type of connection.
One example of a quick-connect/disconnect pipe coupling not specifically related to couplings to be used in field of drilling and completing well bores is disclosed in U.S. Pat. No. 5,403,043 to Smet. The connection is made-up of a pin and a box. The pin and the box are provided with interlocking fins and have axial slots between the fins allowing fins of the other coupling part to pass through in axial direction when a coupling is made or released. To make a connection, the pin is axially inserted in the box causing the fins of the pin and the box to pass in axial direction through slots left open between the fins of the other coupling part. Subsequently, the coupling parts are rotated relative to each other over in this example one eighths of a turn, causing the fins to engage each other. Because the fins do not engage during insertion and the coupling members have to be rotated over a relatively small angle only, the coupling can in principle be made very quickly.
However, a disadvantage of couplings having cooperating fins or teeth which are separated circumferentially to allow fins or teeth of the other coupling part to pass by in axial direction is that the load carrying surface of the threads is small compared to a coupling with continuous thread completely circumventing the coupling members. This in turn entails disadvantages in terms of the required size of the coupling, durability and reliability. These are important disadvantages in many applications, notably in the field of subterranean drilling.
In one embodiment, the coupling members are provided with saw tooth shaped teeth on the distal end face of the pin and on the bottom of the box. These tooth shaped teeth pass each other when a connection is made-up or released. This, in turn causes the pin to be pushed out of the box causing a substantial loading of the interlocking fins.
In U.S. Pat. No. 3,895,829 to Manson, Jr. a coupling is disclosed of which a pin member and a box member are each provided with interrupted threads. The threads are arranged so that the pin member can readily be fitted into the box member or removed therefrom when the threads are disengaged. It is preferred that the threads are pitched slightly to minimize the torque required to disengage the threads and to distribute tension loads uniformly to all threads. However, the thread lead can be counterclockwise or clockwise, or threads with no lead can be used. To lock the coupling in connected condition, an axially movable locking sleeve is provided which in one position engages both the box member and the pin member so that rotation of these members relative to each other is precluded. This coupling is of a relatively complicated design and specifically suitable for use in situations where the coupling needs to be disconnected remotely in the course of drilling or completing a well bore in subterranean formations.
U.S. Pat. No. 4,688,832 to Ortloff et al. discloses a pipe coupling with a pin and box member which, in contrast to the previously discussed couplings, are provided with continuous threads. When the coupling is in hand-tight condition, the clearances between sections of the continuous threads of the pin that are adapted to mate with selected sections of threads of the box are different for different sections of the thread. Thus, in made-up condition and under external loads a more uniform distribution of loads is obtained. However, during relative rotation of the coupling members from a hand-tight condition to a fully made-up condition some section are loaded during a larger portion of the rotation than other sections. During disconnection of the coupling, the sections which have engaged last during make-up disengage first. This entails that some sections wear more quickly than others, which is disadvantageous for the precisely applied differences in clearance and reduces the number of times a coupling can be connected and disconnected before reconditioning of the coupling is required.
U.S. Pat. Reissue No. 30,647 issued to Blose discloses a pipe coupling with a pin and box member provided with respectively matching continuous dove-tail threads. On the pin and box members the thread has a smaller pitch distance at the thread tip than at the thread root so that the threads are formed by helical wedges. When made-up the interlocking wedges mutually squeeze each other in axial direction so that the coupling can withstand the application of a large torque and the threads are functionally leak resistant. However, if axial pre-stress loads are applied, this does not contribute to distributing the axial pre-stress loads more evenly over different sections of the threads.